The present disclosure relates to a dilatometer for measuring metallic samples.
There are dilatometers in which a metallic sample is clamped between two clamping pins in a longitudinal direction. The sample is heated up by an induction coil and the change in length is measured with the aid of the two clamping pins. In addition, the sample temperature is detected in a central region of the sample. A disadvantage of such a dilatometer is that heat is transferred out of the sample at the surfaces that are in contact with the clamping pins, thus resulting in a temperature gradient within the sample and so affecting the correlation between the measured temperature and the change in length. Particularly in the case of phase transformations being examined, this can lead to them not running simultaneously within the sample because the phase transformation, for example, in a central region, is already completed while it is just commencing in the peripheral regions. Asymmetries of the induction field additionally affect these temperature gradients negatively. In the case of such dilatometers, the measurements are afflicted with errors since the phase transition cannot be detected flawlessly, which is especially noticeable in the case of high heating up and cooling down rates.
The present disclosure includes a dilatometer for measuring metallic samples by which an exact measurement of the temperature-dependent change in length is achieved.
The exact measurement is achieved by a dilatometer that includes a sample holder configured to receive and clamp a sample, an induction coil arranged on the sample, the induction coil configured to heat the sample, a sensor for measuring the temperature of the sample, and an optical measuring device for detecting a change in a length of the sample.
As noted above, the dilatometer of the present disclosure includes an optical measuring device for detecting a change in length of the sample. This makes it possible to undertake a contact-free measurement of a change in length. Due to this contact-free measurement, no heat outflow takes place at the measuring point. Moreover, with the optical measuring device, the change in length can be detected with high accuracy.
According to an embodiment of the present disclosure, the optical measuring device detects a change in length in a measuring plane and the sensor for measuring the temperature of the sample measures the temperature in the region of this plane. As a result, an exact measurement of change in length can be carried out even in the case of a temperature gradient presenting in a longitudinal direction. That is because the measurement of the temperature takes place in the region of the measurement of change in length. Measuring the “length change” in this context does not necessarily mean a measurement in the longitudinal direction of a sample, but rather in any direction of the sample, for example, transverse to a longitudinal direction of the sample. The sensor can detect the temperature in the region of the measuring plane of the optical measuring device, wherein within the region a slight space between the sensor and the measuring plane is also covered. The slight space not adversely affecting the measurement.
According to a further embodiment of the present disclosure, the optical measuring device has a transmitter and a receiver and at least one sensor for measuring the temperature, which is situated in the shadow of the optical measuring device at the side of the sample facing towards or facing away from the receiver. In this way, a temperature measurement can take place exactly in the measuring plane, wherein the sensor does not interfere with the optical measuring device as it is situated in the shadow or at the side facing the transmitter. Only those peripheral regions of the sample being measured by the optical measuring device should be kept away from the sensor.
In the case of the optical measuring device, a light source with a collimator for producing a parallel beam path can be employed as a transmitter, according to the present disclosure. As a receiver, sensors can be used to detect the silhouette image of the sample, for example a high-speed linear CCD sensor. The sensor for measuring the temperature can therefore make contact with the sample in the “shadowed area” of the optical measuring device and so does not hinder the measurement of a change in length.
In an embodiment according to the present disclosure, the sensor is designed as a thermocouple that is fixed at the tip to the sample. Because of the small cross-section of the conductors of the thermocouples, these do not affect the temperature of the sample in any lasting manner.
The at least one induction coil may, for example, have a gap in a central region, and a measuring plane of the optical measuring device is situated in the region of this gap. This way, the measuring plane can be oriented perpendicular to a longitudinal direction of the sample, wherein merely the gradient of the induction coil in the region of the gap is increased slightly in order to provide space for a measuring plane of the optical measuring device. The at least one induction coil can, in accordance with the present disclosure, have arbitrary shapes. For example, circular coils are used, but also flat coils and specially adapted coils, which have a gap in the central region for the optical measuring device so that the measuring beam can detect the change in length of the sample without hindrance. Additionally, it is within the scope of the present disclosure to provide a helical induction coil with a uniform winding and to align the measuring plane essentially parallel to the gradient of the winding, so that the arrangement of a broader gap in the region of the induction coil can be dispensed with.
The sample holder may, for example, include two clamping pins and/or transmission rods, between which the sample can be clamped in a longitudinal direction. By way of the transmission rods, a length change in a longitudinal direction of the sample can also be detected. In addition, the optical measuring device can then carry out a measurement essentially perpendicular to the longitudinal direction.
Other aspects of the present disclosure will become apparent from the following descriptions when considered in conjunction with the accompanying drawings.